Substituted α-pyrones

ABSTRACT

α-Pyrones of the formula I ##STR1## in which R 1  is --CF 3  or --COOR 3  and R 3  is the radical of a C 1  -C 18  alcohol diminished by a hydroxyl group, and R 2  is --F, --Br, --Cl, --CN, --CF 3 , C 1  -C 18  alkyl, C 2  -C 18  alkenyl, C 2  -C 18  alkynyl, C 1  -C 18  alkoxy, C 1  -C 18  alkylthio, C 1  -C 18  alkylsulfonyl, C 6  -C 16  aryl, C 7  -C 24  alkaryl, C 7  -C 12  aralkyl, C 8  -C 24  alkaralkyl, C 6  -C 16  aryloxy, C 6  -C 16  arylthio, C 6  -C 16  arylsulfonyl, C 7  -C 24  alkaryloxy, C 7  -C 24  alkarylthio, C 7  -C 24  alkarylsulfonyl, C 7  -C 12  aralkyloxy, C 7  -C 12  aralkylthio, C 7  -C 12  aralkylsulfonyl, C 8  -C 24  alkaralyloxy, C 8  -C 24  alkaralylthio, C 8  -C 24  alkaralylsulfonyl, secondary amine having 2 to 24 C atoms or trialkylsilyl or trialkoxysilyl each of which has 3 to 18 C atoms. They are suitable for the preparation of naphtho-1,4-quinones and anthra-1,4-quinones, from which tetrathiotetracenes or tetraselenotetracenes having electrochromic properties can be obtained.

The invention relates to 4-substituted α-pyrone-5-carboxylic acid estersor 5-trifluoromethyl-α-pyrones and to 7-substitutednaptho-1,4-quinone-6-carboxylic acid esters or6-trifluoromethylnaphtho-1,4-quinones and to substitutedbutadienedicarboxylic acid esters.

α-Pyrone-5-carboxylic acid esters are known, see, for example, H. Gaultet al, C.r. Acad. Sci. Paris Ser. C, 266. pages 131-134 (1968).α-Pyrone-4,5-dicarboxylic acid diesters are described in U.S. Pat. No.4,617,151.

Other α-pyrone-5-carboxylic acid esters which are unsubstituted in the3,6-position and substituted in the 4-position are not known. It has nowbeen found that α-pyrones of this type can be obtained in a simplemanner by reacting allenedicarboxylic acid esters with organometalliccompounds to give propenedicarboxylic acid diesters which aresubstituted in the 2-position, reacting the latter with formic acidesters to give 1-alkoxymethylenebutadiene-2,4-dicarboxylic acid esterswhich are substituted in the 3-position and cyclizing the latter to giveα-pyrones.

The invention relates to compounds of the formula I ##STR2## in which R¹is --CF₃ or --COOR³ and R³ is the radical of a C₁ -C₁₈ alcoholdiminished by a hydroxyl group, and R² is --F, --Br, --Cl, --CN, --CF₃,C₁ -C₁₈ alkyl, C₂ -C₁₈ alkenyl, C₂ -C₁₈ alkynyl, C₁ -C₁₈ alkoxy, C₁ -C₁₈alkylthio, C₁ -C₁₈ alkylsulfonyl, C₆ -C₁₆ aryl, C₇ -C₂₄ alkaryl, C₇ -C₁₂aralkyl, C₈ -C₂₄ alkaralkyl, C₆ -C₁₆ aryloxy, C₆ -C₁₆ arylthio, C₆ -C₁₆arylsulfonyl, C₇ -C₂₄ alkaryloxy, C₇ -C₂₀ alkarylthio, C₇ -C₂₄alkarylsulfonyl, C₇ -C₁₂ aralkyloxy, C₇ -C₁₂ aralkylthio, C₇ -C₁₂aralkylsulfonyl, C₈ -C₂₄ alkaralkyloxy, C₈ -C₂₄ alkaralkylthio, C₈ -C₂₄alkaralkylsulfonyl, secondary amine having 2 to 24 C atoms ortrialkylsilyl or trialkoxysilyl each of which has 3 to 18 C atoms.

In a preferred subgroup, R² is --F, --Br, --Cl, --CF₃, C₁ -C₁₈ alkyl, C₂-C₁₈ alkenyl, C₁ -C₁₈ alkoxy, C₁ -C₁₈ alkylthio, C₁ -C₁₈ alkylsulfonyl,C₆ -C₁₀ aryl, C₇ -C₂₄ alkaryl, C₇ -C₁₂ aralkyl, C₆ -C₁₀ aryloxy, C₆ -C₁₀arylthio, C₆ -C₁₀ alkylsulfonyl, C₇ -C₁₂ aralkyloxy, C₇ -C₁₂ aralkylthioor C₇ -C₁₂ aralkylsulfonyl.

As the radical of an alcohol, R³ preferably contains 1-12 C atoms,especially 1 to 6 C atoms. R³ can, for example, be linear or branchedalkyl, C₅ cycloalkyl or C₆ cycloalkyl each of which is unsubstituted orsubstituted by C₁ -C₆ alkyl, or C₆ -C₁₂ aryl or C₆ -C₁₂ aryl-C_(x)H_(2x) each of which is unsubstituted or substituted by C₁ -C₆ alkyl andin which x is 1 to 4. Aryl is preferably phenyl and x is preferably 1.R³ is preferably C₁ -C₁₈ alkyl, particularly C₁ -C₁₂ alkyl andespecially C₁ -C₆ alkyl. Some examples are methyl, ethyl, the isomers ofpropyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl,dodecyl, tetradecyl, hexadecyl and octadecyl, cyclopentyl, cyclohexyl,methylcyclohexyl, phenyl, benzyl, methylphenyl, dimethylphenyl,ethylphenyl, methylethylphenyl, t-butylphenyl and methylbenzyl. It isparticularly preferable for R³ to be methyl or ethyl.

As alkyl, alkoxy, alkylthio and alkylsulfonyl, R² can be linear orbranched and preferably contains 1 to 12 C atoms, particularly 1 to 6 Catoms and especially 1 to 4 C atoms. Examples are methyl, ethyl and theisomers of propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl,undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, heptadecyl andoctadecyl and the corresponding alkoxy, alkylthio and alkylsulfonylradicals.

As alkenyl and alkynyl, R² can be linear or branched and preferablycontains 2 to 12 C atoms, particularly 2 to 6 C atoms. Some examples arevinyl, ethynyl, allyl, propargyl, prop-1-en-1-yl, but-1-en-4-yl,but-2-en-4-yl, pent-2-en-5-yl, hex-3-en-6-yl, but-2-yn-4-yl,pent-2-yn-4-yl, pent-2-yn-5-yl, hex-3-yn-5-yl and hex-3-yn-6-yl. R² asaryl or as a radical containing aryl is, for example, naphthyl orparticularly phenyl. R² as alkaryl and the corresponding oxy, thio andsulfonyl radicals preferably contain 7 to 18 C atoms. R² as aralkyl andthe corresponding oxy, thio and sulfonyl radicals preferably contain 7or 8 C atoms. R² as alkaralkyl and the corresponding oxy, thio andsulfonyl radicals preferably contain 8 to 20 C atoms. In a preferredsubgroup, R² as aryl or as a radical containing aryl is phenyl, phenoxy,phenylthio or phenylsulfonyl, C₁ -C₁₂ alkylphenyl, C₁ -C₁₂ alkylphenoxy,C₁ -C₁₂ alkylphenylthio or C₁ -C₁₂ phenylsulfonyl, C₁ -C₁₂alkylphenyl--C_(n) H_(2n) --, C₁ -C₁₂ alkylphenyl --C_(n) H_(2n) O--, C₁-C₁₂ alkylphenyl--C_(n) H_(2n) S-- or C₁ -C₁₂ alkylphenyl--C_(n) H_(2n)SO₂ --, or phenyl--C_(n) H_(2n) --, phenyl--C_(n) H_(2n) O--,phenyl--C_(n) H_(2n) S-- or phenyl--C_(n) H_(2n) SO₂ -- in which n is anumber from 1 to 4, in particular 1 or 2 and especially 1. Some examplesare phenyl, phenoxy, phenylthio, benzyl, benzyloxy, benzylthio,methylphenyl, methylphenoxy, methylphenylthio, methylbenzyl,methylbenzyloxy and methylbenzylthio.

As secondary amino, R² preferably contains 2 to 18, in particular 2 to12, and especially 2 to 8, C atoms. The secondary amino can have theformula --NR⁵ R⁶ in which R⁵ and R⁶ independently of one another arelinear or branched C₁ -C₁₈ alkyl, in particular C₁ -C₁₂ alkyl andespecially C₁ -C₆ alkyl, cyclopentyl, cyclohexyl, phenyl, benzyl, C₁ -C₆alkylphenyl or C₁ -C₆ alkylbenzyl, or R⁵ and R⁶ together aretetramethylene, pentamethylene or 3-oxa-1,5-pentylene. Some examples aredimethylamino, diethylamino, ethylmethylamino, dibutylamino,dioctylamino, methyldodecylamino, didodecylamino, methyloctadecylamino,phenylmethylamino, benzylmethylamino, pyrrolino, piperidino andmorpholino.

As trialkylsilyl or trialkoxysilyl, R² preferably contains 3 to 12 Catoms, in particular 3 to 8 C atoms. Alkyl and alkoxy groups have beenmentioned above. Some examples are trimethylsilyl, triethylsilyl,dimethylethylsilyl, tributylsilyl, dimethylbutylsilyl,dimethyl-(1,1,2,2-tetramethylethyl)silyl, dimethyloctylsilyl,trimethoxysilyl and triethoxysilyl.

A preferred group of compounds according to the invention is constitutedby those in which R² is --F, --CN, --CF₃, --Br, --Cl, C₁ -C₁₈ alkyl orC₂ -C₁₂ alkenyl, C₁ -C₁₈ alkoxy, C₁ -C₁₈ alkylthio, C₁ -C₁₈alkylsulfonyl, phenyl, C₁ -C₁₂ alkylphenyl, phenyl--C_(n) H_(2n) --, C₁-C₁₂ alkylphenyl--C_(n) H_(2n) --, phenoxy, phenylthio orphenylsulfonyl, C₁ -C₁₂ alkylphenoxy, C₁ -C₁₂ alkylphenylthio or C₁ -C₁₂alkylphenylsulfonyl, phenyl--C_(n) H_(2n) --O--, phenyl--C_(n) H_(2n)--S-- or phenyl--C_(n) H_(2n) --SO₂ --, C₁ -C₁₂ alkylphenyl--C_(n)H_(2n) --O--, C₁ -C₁₂ alkylphenyl--C_(n) H_(2n) --S-- or C₁ -C₁₂alkylphenyl--C_(n) H_(2n) --SO₂ --, in which n is a number from 1 to 4,secondary amino having 2 to 18 C atoms, or trialkylsilyl ortrialkoxysilyl having 3 to 12 C atoms.

In a particularly preferred embodiment, R² is --F, --Cl, --Br, --CF₃, C₁-C₁₂ alkyl, C₁ -C₁₂ alkoxy, C₁ -C₁₂ alkylthio, benzylthio or C₁ -C₁₂alkylbenzylthio or secondary amino having 2 to 12 C atoms.

The invention also relates to a process for the preparation of compoundsof the formula I, which comprises cyclizing a compound of the formula II##STR3## in which R² and R³ are as defined above and R⁴ is C₁ -C₄ alkyl,in the presence of a strong, anhydrous acid, to give a compound of theformula I.

In formula II, R⁴ is particularly methyl or ethyl.

The invention also relates to the compounds of the formula II. They canbe prepared in a manner analogous to the process described in U.S. Pat.No. 4,617,151 by reacting propene-1,3-dicarboxylic acid diesters of theformula III ##STR4## with formic acid esters HCOOR⁴ in the presence ofTiCl₄ or an alkali metal hydride or alcoholate. Examples of suitablealkali metals are Li, Na and K and Examples of alcohols are C₁ -C₄alkanols, for example methanol and ethanol.

The compounds of the formula III can be prepared by various methods. Thestarting compound is, for example, the known ketodicarboxylic aciddiester of the formula IV ##STR5## Compounds of the formula III in whichR² is an alkoxy radical are obtained by enolization with alkylatingagents, for example dialkyl sulfates, orthoformic acid esters in thepresence of FeCl₃, or alkyl halides.

The reaction of the compound of the formula IV with, for example, PCl₅or diethylaminosulfur trifluoride (DAST) gives compounds of the formulaIII in which R² is Cl or F. These can be reacted with alkali metal saltsof organic mercapto compounds or sulfonyl compounds (examples of alkalimetal are Li, Na or K) to give compounds of the formula III in which R²is one of the thio radicals or sulfonyl radicals defined above.

Compounds of the formula III in which R² is Cl can be dehydrochlorinatedin a customary manner to give allenedicarboxylic acid esters of theformula V ##STR6## Reacting the latter with NH₄ HF₂ or (n-butyl)₄ NH₂ F₃also leads to compounds of the formula III in which R² is F. Compoundsof the formula III are obtained by means of organometallic reagents R² Z(R² is not halogen or --CF₃) in which Z is preferably an alkali metal,in particular Li, Na or K, followed by hydrolysis. If R² is ahydrocarbon radical, it is preferable to use an inorganic copper salt,for example copper(I) halides or in particular CuCN, concomitantly inthe reaction with R² Z. Compounds of the formula III substituted bysecondary amines are also obtained by direct reaction with secondaryamines. The reaction with R² Z is advantageously carried out in a polar,aprotic solvent. Examples are ethers, especially diethyl ether, dibutylether, tetrahydrofuran, dioxane or ethylene glycol diethyl ether. Thereaction temperature is advantageously -20° to -100° C.

Compounds of the formula I in which R¹ and/or R² are --CF₃ are obtainedby fluorinating, for example using SF₄ /HF, the corresponding carboxylicacid alkyl esters, in particular the methyl or ethyl ester.

Compounds of the formula I in which R² is a thio or sulfonyl radical canalso be prepared by appropriate substitution of α-pyrones of the formulaI in which R² is Cl or Br.

The cyclization of compounds of the formula II can also be carried outby the process described in specification U.S. Pat. No. 4,617,151. It isadvantageous to carry out the cyclization using anhydrous formic acid orpolyphosphoric acid without a solvent. The temperature for this reactionis advantageously 50° to 250° C.

The compounds of the formula I can be reacted with benzo-1,4-quinone togive naphthoquinones of the formula VI ##STR7## in which R¹ and R² areas defined in formula I. The invention also relates to thesenaphthoquinones.

Preferably, R² in formula VI is --CF₃, F, --Cl, --Br, C₁ -C₁₈ alkyl, C₁-C₁₈ alkoxy, C₁ -C₁₈ alkylthio, C₁ -C₁₈ alkylsulfonyl, phenoxy,phenylthio, phenylsulfonyl, C₁ -C₁₂ alkylphenoxy, C₁ -C₁₂alkylphenylthio, C₁ -C₁₂ alkylphenylsulfonyl, benyloxy, benzylthio,benzylsulfonyl, C₁ -C₁₂ alkylbenzyloxy, C₁ -C₁₂ alkylbenzylthio or C₁-C₁₂ alkylbenzylsulfonyl. In a particular embodiment, R¹ is --CH₃ or--COOR³ and R² is --CF₃, --F, --Cl, C₁ -C₁₂ alkyl, C₁ -C₁₂ alkoxy, C₁-C₁₂ alkylthio, C₁ -C₁₂ alkylsulfonyl or benzylthio and R³ is C₁ -C₄alkyl.

The compounds of the formula I and VI are valuable intermediates for thepreparation of substituted anthracene-5,12-diones, from whichsubstituted tetrathiotetracenes and/or tetraselenotetracenes can beobtained (cf. U.S. Pat. No. 4,617,151). Electrically conductivecharge-transfer complexes (CT complexes) can be prepared from suchchalkogenated tetracenes by means of electron acceptors. They can beattached to polymers, for example incorporated into polymers as sidegroups (cf. U.S. Pat. No. 4,617,151) by means of their functionalgroups. The CT complexes are also suitable for the production of, forexample, antistatic coatings of photographic film elements, magnetictapes, electrophotographic film elements and electronic components (seeU.S. Pat. No. 3,634,336). The chalkogenated tetracenes also exhibitelectrochromic properties; they can be used for electrochromic displays.They are also suitable for use as laser-optical data storage [Nach.Chem. Techn. Lab. 35, pages 255 et seq. (1987)] and as anode material inorganic solid state batteries (EP-A 0,090,598). CT complexes ofunsubstituted or substituted tetrathiotetracenes ortetraselenotetracenes can also be incorporated into thermoplastic,thermosetting or elastomeric polymers in order to achieve antistaticproperties. This is advantageously effected, for example, by dissolvingthese tetrathiotetracenes or tetraselenotetracenes, together with asoluble polymer or a precursor thereof and an electron acceptor, forexample an agent which forms halogen (organic halogenated compounds, forexample bromoform, trichlorobromomethane, tetrabromomethane,hexachloropropane, perchlorobutadiene, 1,3-dichloro-2-butene,1,4-dichloro-2-butene, 1,4-bis-(trichloromethyl)-benzene,iodoacetonitrile, iodoform, tetrachloroethylene,perchlorocyclobutadiene, N-chlorosuccinimide, N-bromosuccinimide orN-iodosuccinimide), if appropriate together with a further inertsolvent, and removing by evaporation at an elevated temperature theexcess agent which forms halogen and the solvent. The composition formedcontains, in the polymer, a network of needle-shaped crystals of the CTcomplex, if the chalkogenated tetracene is unsubstituted or containssmall substituents (for example F, CH₃ or CF₃). Compositions of thistype exhibit a high electrical conductivity. This can be improvedfurther if a substituted tetrathiotetracene or tetraselenotetraceneprepared from the compounds of the formula I and which does not formsuch a network and which is present in finely divided form in thepolymer matrix is concomitantly used, since substitutedtetrathiotetracenes or tetraselenotetracenes of this type have notendency or only a slight tendency to crystallize in the polymer.

The compounds of the formula I are valuable diene components forDielsAlder reactions, for which a variety of dieneophiles can be used.If substituted or unsubstituted 1,4-quinones are used, for examplebenzo-1,4-quinone, naphthalene-1,4-quinone or anthracene-1,4-quinone,quinones enlarged by a ring are obtained (see, for example, EP-A0,195,743). Substituted naphthacene-5,12-diones can also be obtaineddirectly by reacting substituted or unsubstitutednaphthalene-1,4-quinones with benzocyclobutenes which are substituted inthe 1-position and 2-position by bromine and/or iodine. Anthraquinonesand naphthacene diones can be used, for example, as photosensitizers(cf. U.S. Pat. No. 3,941,759) or photoinitiators for the polymerizationof ethylenically unsaturated compounds, in particularnaphthacene-5,12-diones which are at least substituted by a thio radicalin the 2-position. Naphthacene quinones can also be used inelectrochromic display elements (Japanese Preliminary PublishedSpecification 61-43,680).

The following examples illustrate the invention.

EXAMPLE 1 Ethyl 4-butyl-2H-2-oxopyran-5-carboxylate (a) Diethyl2-butylprop-1-ene-1,3-dicarboxylate

10 g of CuCN are suspended in 200 ml of toluene and the toluene isremoved by distillation at 35° C. on a rotary evaporator. The procedureis repeated again. The vacuum is replaced by argon, 200 ml oftetrahydrofuran (THF) are added and the suspension is cooled to -70° C.125 ml of a hexane solution of 12.8 g of n-butyllithium are addeddropwise at this temperature. A yellow solution is formed, with anexothermic reaction.

After 15 minutes of stirring, 13.8 g of diethyl allenedicarboxylate in100 ml of THF are added dropwise at -50° C. A red solution is formed,which is stirred for a further 2 hours at -20° C. After 2 hours thereaction mixture is warmed to room temperature. 500 ml of a buffersolution (27 g of NH₄ Cl dissolved in 100 ml of concentrated ammoniasolution) are then added to the reaction mixture. The reaction mixtureis stirred at room temperature for 15 minutes, diluted with 500 ml ofsaturated NaCl solution and extracted twice with 400 ml of diethylether. The combined extracts are dried by means of Na₂ SO₄ and thediethyl ether is removed by distillation on a rotary evaporator. Thedistillation residue (16.7 g) is chromatographed over 300 g of SiO₂(mobile phase CH₂ Cl₂ ; 0.3-0.5 bar excess pressure): yield 12.94 g.

Distillation of the product in a kugelrohr oven at 120° C./1.3×10⁻² mbargives a colourless oil.

(b) Diethyl 2-butyl-3-ethoxymethylenylprop-1-ene-1,3-dicarboxylate

11.34 g (0.06 mol) of TiCl₄ in 15 ml of CCl₄ are added dropwise, withstirring and at 0° C., to 120 ml of THF. After 15 minutes 4.44 g (0.06mol) of ethyl formate followed by 3.63 g (0.015 mol) of diethyl2-butylprop-1-ene-1,3-dicarboxylate are added to the resulting yellowcomplex. After 15 minutes of stirring, 12.12 g (0.012 mol) ofN-methylmorpholine in 21 m of THF are added dropwise and in the courseof 30 minutes to the reaction mixture at 0°-5° C. The reaction mixtureis stirred at room temperature for 24 hours and is then introduced into150 ml of CH₂ Cl₂ and 100 ml of saturated KHCO₃ solution. After stirringfor 30 minutes, the precipitated titanium salt is filtered off. The CH₂Cl₂ solution is separated from the water in the filtrate. The CH₂ Cl₂phase is washed with water, dried with Na₂ SO₄ and evaporated, andresidual N-methylmorpholine is removed at 100° C./1.3 mbar. The oilyresidue is filtered through 150 g of SiO₂ (mobile phase CH₂ Cl₂ ; 0.3bar excess pressure). The crude product obtained is chromatographed(silica gel; mobile phase: CH₂ Cl₂). 1.95 g (43.5%) of a pale yellow oilare obtained.

(c) Ethyl 4-butyl-2H-2-oxopyran-5-carboxylate

1.4 g (0.0047 mol) of diethyl2-butyl-3-ethoxymethylenylprop-1-ene-1,3-dicarboxylate and 14 ml offormic acid are heated for 20 minutes in a bath preheated to 110° C. Theformic acid is then removed by distillation at 50° C. under a water pumpvacuum. The residue is dissolved in 20 ml of CH₂ Cl₂, and the solutionis washed with 10 ml of saturated NaCl solution, dried with Na₂ SO₄ andevaporated to dryness. Yield: 0.95 g (90.5%) of an oily product.

EXAMPLE 2 Ethyl 4-benzylthio-2H-2-oxopyran-5-carboxylate (a) Diethyl2-benzylmercaptoprop-1-ene-1,3-dicarboxylate

1.84 g of Na are dissolved in 200 ml of methanol. 10.91 g of benzylmercaptan are added, and the methanol is removed by distillation invacuo. The distillation residue is evaporated twice with 100 ml oftoluene. The residue is dissolved in 120 ml of dimethyl sulfoxide(DMSO), and 17.64 g of diethyl β-chloroglutaconate are added to thesolution. The reaction mixtures warms up to 53° C. It is stirredovernight at room temperature. The DMSO is removed by distillation invacuo at 80° C./1.3 mbar. The residue is taken up in CH₂ Cl₂, and thesolution is washed with acidified (HCl) water. Drying with Na₂ SO₄ andremoving the CH₂ Cl₂ by distillation gives 24.15 g of product (97.9% inthe form of a golden-yellow oil.

(b) Diethyl2-benzylmercapto-3-ethoxymethylenoprop-1-ene-1,3-dicarboxylate

105.2 g (0.557 mol) of TiCl₄ in 140 ml of CCl₄ are added dropwise, withstirring and at 0° C., to 1115 ml of THF. First 41.2 g (0.557 mol) ofethyl formate and then 43 g (0.139 mol) of diethyl2-benzylmercaptoprop-1-ene-1,3-dicarboxylate are added at the sametemperature. After 15 minutes a solution of 112.9 g (1.115 mol) ofN-methylmorpholine in 195 ml of THF is added dropwise in the course of30 minutes at the same temperature. The reaction mixture is then stirredat room temperature overnight and is then introduced into a mixture of1000 ml of CH₂ Cl₂ and 2000 ml of saturated NaHCO₃ solution. After beingstirred for 30 minutes, the mixture is filtered. The CH₂ Cl₂ phase ofthe filtrate is separated from the aqueous phase, washed again twicewith 300 ml of water, dried with Na₂ SO₄ and concentrated in vacuo.47.66 g (94%) of an orange oil are obtained.

(c) Ethyl 4-benzylthio-2H-2-oxopyran-5-carboxylate

47.66 g (0.130 mol) of diethyl2-benzylmercapto-3-ethoxymethylenylprop-1-ene-1,3-dicarboxylate aredissolved in 481 ml of formic acid and the mixture is heated for 20minutes in a bath preheated to 110° C. The formic acid is then removedby vacuum distillation. The residue is taken up in 500 ml of CH₂ Cl₂ andis washed with aqueous NaCl solution and then with water. After thesolution has been dried with Na₂ SO₄, the solvent is removed by vacuumdistillation. The distillation residue crystallizes out [25.9 g(68.7%)]. The melting point (recrystallized from ethanol) is 99°-101° C.

EXAMPLE 3 Ethyl 4-chloro-2H-2-oxopyran-5-carboxylate (a) Diethyl1-ethoxy-3-chlorobutadiene-2,4-dicarboxylate

7.56 g (0.04 mol) of TiCl₄, dissolved in 10 ml of CCl₄, are addeddropwise at 0° C. to 80 ml of THF. First 2.96 g (0.04 mol) of ethylformate and then 2.2 g (0.01 mol) of β-chloroglutaconic acid are addedto the resulting yellow complex. After 15 minutes a solution of 8.08 g(0.08 mol) of N-methylmorpholine in 14 ml of THF is added dropwise at 0°C. in the course of 30 minutes. The reaction mixture is stirredovernight at room temperature. The solvent is then removed from thereaction mixture by vacuum distillation at 35° C. The residue issuspended in 200 ml of CH₂ Cl₂, 200 ml of water containing 11 g ofNaHCO₃ are added and the mixture is filtered after being stirred for 1hour. The two phases of the filtrate are separated and the organicsolution is concentrated under a water pump vacuum. The residue isfiltered through a column of 35 g of silica gel (mobile phase: CH₂ Cl₂).1.92 g (69.6%) of an oily product remain after the mobile phase has beenremoved by distillation.

(b) Ethyl 4-chloro-2H-2-oxopyran-5-carboxylate

74.12 g (0.268 mol) of diethyl1-ethoxy-3-chlorobutadiene-2,4-dicarboxylate, dissolved in 700 ml offormic acid, are heated for 30 minutes in a bath preheated to 110° C.,and are cooled and evaporated to dryness in vacuo at 50° C. The residueis taken up in 250 ml of CH₂ Cl₂, the solution is washed with wateruntil neutral, dried with Na₂ SO₄ and then concentrated. Thedistillation residue is chromatographed over 1300 g of silica gel(mobile phase: 99:1 CH₂ Cl₂ /acetone, 0.3 bar excess pressure). Afterconcentration, the distillation residue (29 g) is crystallized from amixture of 12 ml of diethyl ether and 10 ml of hexane at -20° C. Yield22.79 g (42%), melting point: 38°-39° C.

EXAMPLE 4 Ethyl 4-ethoxy-2-oxo-2H-pyran-5-carboxylate (a) Diethyl1,3-ethoxybutadiene-2,4-dicarboxylate

A solution of 130.41 g (0.69 mol) of TiCl₄ in 170 ml of CCl₄ is addeddropwise, with stirring, to 1380 ml of THF. During this addition, thetemperature is kept at 0° C. by cooling. First 36.72 g (0.17 mol) ofethyl 3-ethoxyglutaconate and then 51.06 g (0.69 mol) of ethyl formateare introduced and, finally, 139.38 g (1.38 mol) of N-methylmorpholinein 241 ml of THF are added dropwise at 0° C. in the course of 0.5 hour.The reaction mixture is stirred overnight at room temperature and isthen introduced into a solution of 150 g of NaHCO₃ in 1200 ml of H₂ Oand 1200 ml of CH₂ Cl₂, and the mixture is stirred for 2 hours. Theprecipitated titanium dioxide is filtered off with suction. The filtrateis then extracted with 3×300 ml of methylene chloride. The combinedmethylene chloride extracts are dried with Na₂ SO₄ and concentrated. Thecrude product which remains is chromatographed over a column filled with1300 g of silica gel 60 (mobile phase 19:1 CH₂ Cl₂ /acetone, 0.3 barexcess pressure). 31.9 g (67.6%) of an oily product are obtained afterthe mobile phase has been removed.

(b) Ethyl 4-ethoxy-2-oxo-2H-pyran-5-carboxylate

26 g (0.0908 mol) of diethyl 1,3-ethoxybutadiene-2,4-dicarboxylate and156 g of polyphosphoric acid are heated at 140° C. for 30 minutes, andthe mixture is diluted with 1000 ml of water and extracted first with1000 ml of CH₂ Cl₂ and then with 3 times 200 ml of CH₂ Cl₂. The extractsare washed with saturated NaCl solution until they are at pH 5. Thesolution is then dried with Na₂ SO₄ and evaporated. The distillationresidue is chromatographed over 160 g of silica gel 60 (19:1 CH₂ Cl₂/acetone, 0.3 bar excess pressure). After the mobile phase has beenremoved, 11.3 g (58.7%) of crystals of melting point 86°-89° C. areobtained.

EXAMPLE 5 4,5-Bis-trifluoromethyl-2-oxo-2H-pyran

50 g (0.0208 mol) of diethyl 2-oxo-2H-pyran-4,5-dicarboxylate, 170 g(1.57 mol) of SF₄ and 300 g (15 mol) of HF are heated in an autoclave at150° C. for 10 hours. The reaction mixture is diluted with 500 ml of CH₂Cl₂ and is poured into 1500 ml of water. The pH is adjusted to a valueof 7.5 by adding NaHCO₃ gradually, with stirring. The organic phase isseparated off, dried over Na₂ SO₄ and filtered, and the solvent isremoved by vacuum distillation at 50° C. The distillation residue isextracted five times with 100 ml of n-pentane, and the pentane solutionis concentrated. Distillation at 68°-70° C./13 mbar gives 18.62 g(38.5%) of 4,5-bis-trifluoromethyl-2-oxo-2H-pyran.

EXAMPLE 6 Ethyl 4-fluoro-2H-2-oxopyran-5-carboxylate (a) Diethyl3-fluoropent- 2(E)-enedicarboxylate

A mixture of tetrabutylammonium dihydrogentrifluoride (prepared from 30mmol of tetrabutylammonium fluoride, KF and HF), 20 ml of1,2-dichloroethane and 15 mmol (2.76 g) of diethyl allenedicarboxylateis heated under reflux for 1 hour. The mixture is then diluted withmethylene chloride and washed slowly with ice water, sodium bicarbonateand ammonium chloride solution, dried with Na₂ SO₄ and evaporated. Thecrude product is flash-filtered over 50 g of silica gel using 500 ml of4:1 hexane/ethyl acetate and distilled under a high vacuum. 1.79 g (59%of theory) of the desired product are obtained; boiling point: 50°-55°C./0.06 mbar.

(b) Diethyl-1-ethoxy-3-fluorobutadiene-2,4-dicarboxylate

A mixture of 3.78 g (0.02 mol) of TiCl₄ in 5 ml of CCl₄ is addeddropwise, at 0° C. and with stirring, to 40 ml of tetrahydrofuran (THF).1.02 g (0.005 mol) of diethyl 3-fluoropent-2(E)-enedicarboxylate and1.48 g (0.02 mol) of ethyl formate are then added dropwise. After 15minutes, a solution of 4.04 g (0.04 mol) of N-methylmorpholine in 7 mlof THF is added dropwise at 0°-5° C. The reaction mixture is stirred for17 hours at room temperature and is then stirred into a mixture of 50 mlof saturated KHCO₃ solution and 100 ml of CH₂ Cl₂. The precipitatedtitanium salt is filtered off with suction, and the organic phase isseparated off from the aqueous phase. The aqueous phase is extractedonce more with 100 ml of CH₂ Cl₂, and the combined organic phases aredried with Na₂ SO₄ and concentrated. The distillation residue is freedfrom excess N-methylmorpholine at 85° C./1.3 mbar, and the residue ischromatographed over 150 g of SiO₂ (mobile phase 19:1 CH₂ Cl₂ /acetone).This gives 1 g (77% of theory) of the product (an orange oil).

(c) Ethyl 4-fluoro-2H-2-oxo-pyran-5-carboxylate

6 g of PPA (polyphosphoric acid) are added to 1 g (0.0038 mol) ofdiethyl 1-ethoxy-3-fluorobutadiene-2,4-dicarboxylate, and the mixture isstirred for 30 minutes at 80° C., diluted with 20 ml of water andextracted 3 times with 20 ml of CH₂ Cl₂. The CH₂ Cl₂ phase is washedwith H₂ O, dried (Na₂ SO₄) and concentrated. The distillation residue ischromatographed over silica gel (mobile phase 99:1 CH₂ Cl₂ /acetone).Evaporating the solvent leaves a residue of 0.2 g (28% of theory) of thepure product (an orange oil).

EXAMPLE 7 Ethyl 4-methylsulfonyl-2H-2-oxopyran-5-carboxylate (a) Diethyl3-methylsulfonylpent-2-enedicarboxylate

A mixture of 0.44 g (0.002 mol) of diethyl3-chloropent-2-enedicarboxylate and 0.3 g (0.003 mol) of sodiumsulfinate in 1.5 ml of dimethylformamide (DMF) is stirred at 100° C. for1 hour, and DMF is then distilled off in vacuo at 110° C./1.3 mbar. Theresidue is mixed with 0.5 ml of water. The fraction insoluble in watercrystallizes out and is filtered off with suction and washed with water.The crude crystalline material is chromatographed over 12 g of silicagel (mobile phase 99:1 CH₂ Cl₂ /acetone). This gives 0.43 g (81.44% oftheory) of the product; melting point: 54°-57° C.

(b) Diethyl 1-ethoxy-3-methylsulfonylbutadiene-2,4-dicarboxylate

A solution of 4.53 g (0.024 mol) of TiCl₄ in 6 ml of CCl₄ is addeddropwise to 48 ml of THF with ice cooling (0°-5° C). First 1.77 g (0.024mol) of formic acid ester and then 1.58 g (0.006 mol) of diethyl3-methylsulfonylpent-2-enedicarboxylate are added dropwise at the sametemperature, with stirring. After 30 minutes 4.86 g (0.048 mol) ofN-methylmorpholine in 8.5 ml of THF are added at 0°-5° C. in the courseof 1 hour. The reaction mixture is then stirred for 20 hours at 25° C.,diluted with 100 ml of CH₂ Cl₂ and poured into 30 ml of saturated KHCO₃solution. The titanium salt is filtered off with suction and the organicphase is separated from the filtrate. The aqueous phase is extractedwith a further 50 ml of CH₂ Cl₂. The combined CH₂ Cl₂ extracts arewashed with saturated NaCl solution, dried with Na₂ SO₄ andconcentrated. The distillation residue is chromatographed (200 g ofsilica gel; mobile phase 19:1 CH₂ Cl₂ /acetone). This gives 1.17 g(60.94%) of the product.

(c) Ethyl 4-methylsulfonyl-2-oxo-2H-pyran-5-carboxylate

1.2 g of sodium methylsulfinate are introduced into a solution of 2.02 gof ethyl 4-chloro-2-oxo-2H-pyran-5-carboxylate in DMF, and the mixtureis stirred for 2 hours at room temperature. The reaction mixture isdiluted with CH₂ Cl₂ and extracted with saturated aqueous NaCl solution.The organic phase is dried with Na₂ SO₄ and evaporated. The distillationresidue which has crystallized out is mixed with ether, filtered offwith suction and washed with ether. Yield 1.4 g (56.9% of theory);melting point: 95°-97° C.

EXAMPLE 8 6,7-Bis-trifluoromethyl-1,4-naphthoquinone

23.47 g (0.1011 mol) of 4,5-bis-trifluoromethyl-2-oxo-2H-pyran, 54.62 g(0.5053 mol) of 1,4-benzoquinone and 26.30 g (0.3026 mol) of MnO₂ in 225ml of 1,2-dichlorobenzene are heated at 180° C. for 12 hours. Thereaction mixture is then diluted with 200 ml of CHCl₃ and filteredthrough Hyflo®. After the chloroform has been removed by distillation,the distillation residue is chromatographed over a column packed with1300 g of SiO₂ (mobile phase: first cyclohexane and then CHCl₃ ; 0.3 barexcess pressure). Yield 17.94 g (60.3%), melting point: 91°-94° C.

EXAMPLE 9 Ethyl 6-ethoxy-1,4-naphthoquinone-7-carboxylate

0.42 g (0.002 mol) of ethyl 4-ethoxy-2-oxo-2H-pyran-5-carboxylate, 1.08g (0.01 mol) of 1,4-benzoquinone and 0.54 g (0.0062 mol) of MnO₂ in 4.8ml of dichlorobenzene are heated at 180° C. for 12 hours. The resultingreaction mixture is chromatographed over a column of 30 g of silica gel60 (mobile phase 99:1 CH₂ Cl₂ /acetone, 0.3 bar excess pressure). Thisgives 0.41 g (75.9%) of crystals of melting point 60°-62° C.

USE EXAMPLE (a) Preparation of2,3-di-(trifluoromethyl)-5,6,11,12-tetrathiotetracene

25 mmol of 6,7-bis-trifluoromethyl-1,4-naphthquinone, approx. 37 mmol of1,2-dibromobenzocyclobutene [see J. Am. Chem. Soc. 79, pages 1701 etseq. (1957)] containing a little 1-bromo-2-iodobenzocyclobutene and 100ml of xylene are kept under reflux for 16 hours in a water separator.The reaction mixture is cooled and the precipitate is filtered off andwashed with xylene. The crystalline2,3-di-(trifluoromethyl)-naphthacene-5,12-dione is obtained in a yieldof 5.82 g (71%), melting point>280° C.

1.65 mmol of 2,3-di-(trifluoromethyl)-naphthacene-5,12-dione, 5 ml ofethyl acetate, 4.96 mmol of potassium acetate and 3 ml of aceticanhydride are hydrogenated for 35 minutes at 20°-25° C., with theaddition of 0.1 g of Pd/C (5%). The reaction mixture is filtered and theresidue is washed three times with CH₂ Cl₂. The filtrates are evaporatedand the residue is recrystallized from CHCl₂ /pentane.2,3-Di-(trifluoromethyl)-5,12-diacetoxynaphthacene is obtained in ayield of 0.45 g (70%), melting point 149°-153° C.

0.61 mmol of the diacetoxynaphthacene, 2.43 milliequivalents of S₈ and0.01 mmol of p-toluenesulfonic acid in 35 ml of 1,2,4-trichlorobenzeneare refluxed for 20 hours under a gentle stream of argon in a 100 mlflask equipped with a reflux condenser and a gas inlet tube. Aftercooling, the solvent is removed by evaporation under a high vacuum, theresidue is boiled with hexane, and the black powder is filtered off anddried at 60° C. under a high vacuum. 79% of crude product are obtained.It is sublimed at 190° C. (1.3×10⁻⁴ mbar), to give 75.6 mg (30%) of pure2,3-di-(trifluoromethyl)-5,6,11,12-tetrathiotetracene in the form ofsmall black needles. Mass spectrum: M⁺ =488; λ_(max)(1,2,4-trichlorobenzene): 755 nm.

(b) Electrochromism

1 mg of 2,3-di-(trifluoromethyl)-5,6,11,12-tetrathiotetracene and 100 mgof LiClO₄, dissolved in 5 ml of acetone, are introduced into the anodeside of an electrochromic cell consisting of a Teflon membrane and ananode and cathode made of ITO glass both at a distance of 0.5 mm, and asolution of 1 mg of2,3-di-(trifluoromethyl)-5,6,11,12-tetrathiotetracene perchlorate (CTcomplex, for preparation see U.S. Pat. No. 3,634,336) and 100 mg ofLiClO₄ in 5 ml of acetone are introduced into the cathode side. After avoltage of 2 volts has been applied, the colours change in the course ofa few seconds from green to red-violet on the anode side and fromred-violet to green on the cathode side. The original colours areobtained in both halves of the cell on reversing the polarity of thevoltage. The same effect is observed if nitrobenzene ordimethylformamide is used as the solvent.

What is claimed is:
 1. A compound of the formula I ##STR8## in which R¹is --CF₃ or --COOR³ and R³ is the radical of a C₁ -C₁₈ alcoholdiminished by a hydroxyl group, and R² is --F, --Br, --Cl, --CN, --CF₃,C₁ -C₁₈ alkyl, C₂ -C₁₈ alkenyl, C₂ -C₁₈ alkynyl, C₁ -C₁₈ alkoxy, C₁ -C₁₈alkylthio, C₁ -C₁₈ alkylsulfonyl, C₆ -C₁₆ aryl, C₇ -C₂₄ alkaryl, C₇ -C₁₂aralkyl, C₈ -C₂₄ alkaralkyl, C₆ -C₁₆ aryloxy, C₆ -C₁₆ arylthio, C₆ -C₁₆arylsulfonyl, C₇ -C₂₄ alkaryloxy, C₇ -C₂₀ alkarylthio, C₇ -C₂₄alkarylsulfonyl, C₇ -C₁₂ aralkyloxy, C₇ -C₁₂ aralkylthio, C₇ -C₁₂aralkylsulfonyl, C₈ -C₂₄ alkaralyloxy, C₈ -C₂₄ alkaralylthio, C₈ -C₂₄alkaralylsulfonyl, secondary amine having 2 to 24 C atoms ortrialkylsilyl or trialkoxysilyl each of which has 3 to 18 C atomswherein aryl is phenyl or naphthyl.
 2. A compound according to claim 1,in which R³ is C₁ -C₁₈ alkyl.
 3. A compound according to claim 2, inwhich R³ is methyl or ethyl.
 4. A compound according to claim 1, inwhich R² is --F, --CN, --CF₃, --Br, --Cl, C₁ -C₁₈ alkyl or C₂ -C₁₂alkenyl, C₁ -C₁₈ alkoxy, C₁ -C₁₈ alkylthio, C₁ -C₁₈ alkylsulfonyl,phenyl, C₁ -C₁₂ alkylphenyl, phenyl--C_(n) H_(2n) --, C₁ -C₁₂alkylphenyl--C_(n) H_(2n) --, phenoxy, phenylthio or phenylsulfonyl, C₁-C₁₂ alkylphenoxy, C₁ -C₁₂ alkylphenylthio or C₁ -C₁₂alkylphenylsulfonyl, phenyl--C_(n) H_(2n) --O--, phenyl--C_(n) H_(2n)--S-- or phenyl--C_(n) H_(2n) --SO₂ --, C₁ -C₁₂ alkylphenyl--C_(n) H₂--O--, C₁ -C₁₂ alkylphenyl--C_(n) H_(2n) --S-- or C₁ -C₁₂alkylphenyl--C_(n) H_(2n) --SO₂ --, in which n is a number from 1 to 4,secondary amino having 2 to 18 C atoms, or trialkylsilyl ortrialkoxysilyl having 3 to 12 C atoms.
 5. A compound according to claim1, in which R² is --F, --Cl, --Br, --CF₃, C₁ -C₁₂ alkyl, C₁ -C₁₂ alkoxy,C₁ -C₁₂ alkylthio, benzylthio or C₁ -C₁₂ alkylbenzylthio or secondaryamino having 2 to 12 C atoms.